1. Field of the Invention
The present invention relates to a method of fabricating an all ceramic jacket crown which is placed over a decayed or broken tooth so that the original shape and function of such tooth is restored, wherein the ceramic tooth core is made by an innovative method.
2. Description of Prior Art
Conventionally, tooth crowns used for restoring decayed or broken teeth can be classified into metal ceramic crown and all ceramic jacket crown, depending upon the materials employed.
The artificial crown now widely in use is a metal ceramic crown, more specifically a PFM (porcelain fused metal) crown which is manufactured by melting a ceramic powder on a metallic crown formed of a precious metal core such as gold, silver, platinum or palladium so as to produce a natural tooth color appearance. However, the conventional metal ceramic crown is disadvantageous because of its low esthetic quality and due to side effects and sensitiveness caused by the metallic material employed therein.
A ceramic artificial crown, that is, an all ceramic jacket crown comprises single crown and a tooth mold formed of a ceramic material and is attached to an appropriately prepared tooth substance. Unlike a metal ceramic crown, the color of the all ceramic jacket crown may be easily adjusted to match that of a natural tooth to thereby exhibit a high esthetic quality and accordingly is welcome by many relevant experts due to its biological compatibility and in addition, virtually no side effects have been observed.
An all ceramic jacket crown as described above has many advantages relative to a metal ceramic tooth crown, and a variety of ceramic materials and fabrication steps thereof have been developed based on continued research in an effort to replace the conventional metal ceramic crown. Yet, the characteristically low fracture toughness and low physical strength of ceramics are obstacles in the development of all ceramic jacket crowns.
To solve the above problems, an improved fabrication method of an all ceramic jacket crown having high durability has been introduced. According to U.S. Pat. No. 4,772,436 of Sep. 20, 1988, a slip casting method is disclosed wherein, alumina by itself or a mixture formed by adding a considerable amount of zirconia to alumina is used as a ceramic source material. To make the slip of this method, 12.about.20 g of water is added to 100 g of metal oxide powder and 0.05.about.0.5 g of a stabilizer such as polyvinyl alcohol, acrylic acid, cellulose ester or sodium silicate is added thereto. An acid such as citric acid is added to control the pH of the slip. Prior to its use, the slip is subjected to an ultrasonic treatment under a vacuum state to eliminate bubbles therefrom and then a base structure is formed by coating the slip on a tooth mold which contracts during heat treatment, allowing the formed structure to be easily removed therefrom.
The tooth mold is formed of calcium sulfate hemihydrate (CaSO.sub.4 1/2H.sub.2 O) having an expansion rate of 0.1.about.0.4%, and also may be formed of a mixture of a refractory material such as alumina or silica, and a binding agent such as sodium silicate, ethyl silicate, ammonium sulfate or ammonium acid phosphate. Metallic oxide particles coated on the tooth mold form an infrastructure having a pore-like structure which is formed through a first sintering, and the infrastructure exhibits a contraction rate of less than 0.4% and yet maintains the desired precision by offsetting the dry expansion effects which occurred during the preceding tooth mold fabrication step.
When applied to 3.5 .mu.m particles, the first sintering is carried out at a temperature of 1,050.degree. C..about.1,150.degree. C. for one to three hours, and sintering contraction is minimized by the rapid rate of temperature elevation. Glass impregnation, in which glass is impregnated into the infrastructure obtained by the first sintering, is performed at a similar temperature to that of the first sintering for less than two to four hours so as to prevent contraction resulting from the impregnation, thereby not influencing the structure formed as a result of the first sintering.
Oxides such as boron oxide, lead oxide and vanadium oxide are added to increase the wettability in the glass composition. Oxides such as boron oxide, lead oxide and a lanthanum oxide are added to lower viscosity. Also, the reactivity of the glass with regard to the metal oxide must be neither too strong nor too weak, thus a glass powder which contains alumina or zirconia in an amount slightly less than the saturation of glass vis-a vis those metal oxide at the impregnation temperature is employed. The heat expansion coefficient of glass should be a little less than that of the first-sintered oxide frame structure to allow heat impact resistance of the tooth crown. The heat expansion coefficient of glass is increased when a sodium oxide, a calcium oxide or a lithium oxide is added thereto, but decreased when a silicon oxide or a titanium oxide is added thereto. Here, the weight percentages of the major components of glass are, SiO.sub.2 :20, B.sub.2 O.sub.3 :19, Al.sub.2 O.sub.3 :20, La.sub.2 O.sub.3 :30, CaO:5, TiO.sub.2 :4, and pigment oxide:2.
To impregnate glass into the metal oxide skeleton, glass paste which, when heated to the appropriate temperature, melts and spreads spontaneously inside the entire volume of the infrastructure by filling all of the pores, can be applied on the outer surface of the infrastructure of the tooth prosthesis. In order to prevent the variations in the surface structure caused by the remaining glass, the prosthesis which comes into contact with the tooth should not contact the melted glass and that the filling of the pores located in the vicinity of this surface is effected by capillarity starting from the interior of the infrastructure mass of fritted metal oxide particles.
The structure formed after glass impregnation is treated by one or two enamel layers, thereby obtaining the desired optical characteristics, a variety of colors and resulting in an appearance that is similar to a natural tooth.
To form a ceramic core by a slip casting method, a slip should first be manufactured and a stabilizer should be added thereto for stability, and an ultrasonic treatment has to be applied to make the mixture of the slip to be uniform. The thusly obtained stable and uniform slip is coated on a plaster mold of a tooth using a brush, then dried and sintered at around 1,150.degree. C., and carved out by a knife so that the thickness is adjusted to correspond to the plaster tooth mold. Then, a tooth core is manufactured by impregnating a glass powder into the first-sintered portion. The process for stabilizing, uniformly mixing and drying the slip and for appropriately carving the slip cast to obtain the desired constant core thickness requires a long time and many fabrication steps, and thus, is quite expensive.